AddToTagVisitor: skip insert when a semantically-equal child already exists

Make `AddToTagVisitor` idempotent by default: if `scope` already contains
a child where `SemanticallyEqual.areEqual(existingChild, tagToAdd)` holds,
return without mutating the tree. `SemanticallyEqual` ignores comments
and attribute order, so this catches semantic duplicates that only differ
in whitespace, attribute ordering, or interleaved comments.

Preserves reference identity on no-op runs so that downstream "did this
recipe change anything" checks (dirty-marker tracking, conditional
`doAfterVisit` chains, recipe-cycle convergence) remain accurate.

An `allowDuplicates` opt-out is provided for the rare caller that wants
the prior unconditional-append behavior. Two new constructor overloads
accept the flag without forcing callers to pass `null` for `tagComparator`:
- `(Xml.Tag, Xml.Tag, boolean)`
- `(Xml.Tag, Xml.Tag, @nullable Comparator<Content>, boolean)`

An audit of every in-tree caller (~22 sites in `rewrite-maven`,
`rewrite-csharp`, and `rewrite-xml`) confirmed none want duplicate
sibling tags — Maven POM, csproj, and `AddOrUpdateChild` (which already
gates externally) all rely on the schemas forbidding duplicates or on
caller-side checks that have been redundant until now.

Author: steve-aom-elliott

rewrite-xml/src/main/java/org/openrewrite/xml/AddToTagVisitor.java

@@ -33,19 +33,37 @@ public class AddToTagVisitor<P> extends XmlVisitor<P> {
     @Nullable
     private final Comparator<Content> tagComparator;
 
+    private final boolean allowDuplicates;
+
     public AddToTagVisitor(Xml.Tag scope, Xml.Tag tagToAdd) {
-        this(scope, tagToAdd, null);
+        this(scope, tagToAdd, null, false);
+    }
+
+    public AddToTagVisitor(Xml.Tag scope, Xml.Tag tagToAdd, boolean allowDuplicates) {
+        this(scope, tagToAdd, null, allowDuplicates);
     }
 
     public AddToTagVisitor(Xml.Tag scope, Xml.Tag tagToAdd, @Nullable Comparator<Content> tagComparator) {
+        this(scope, tagToAdd, tagComparator, false);
+    }
+
+    public AddToTagVisitor(Xml.Tag scope, Xml.Tag tagToAdd, @Nullable Comparator<Content> tagComparator, boolean allowDuplicates) {
         this.scope = scope;
         this.tagToAdd = tagToAdd;
         this.tagComparator = tagComparator;
+        this.allowDuplicates = allowDuplicates;
     }
 
     @Override
     public Xml visitTag(Xml.Tag t, P p) {
         if (scope.isScope(t)) {
+            if (!allowDuplicates && t.getContent() != null) {
+                for (Content existing : t.getContent()) {
+                    if (existing instanceof Xml.Tag && SemanticallyEqual.areEqual(existing, tagToAdd)) {
+                        return super.visitTag(t, p);
+                    }
+                }
+            }
             assert getCursor().getParent() != null;
             if (t.getClosing() == null) {
                 t = t.withClosing(autoFormat(new Xml.Tag.Closing(Tree.randomId(), "\n",

rewrite-xml/src/test/java/org/openrewrite/xml/AddToTagTest.java

@@ -143,6 +143,102 @@ public Xml visitDocument(Xml.Document x, ExecutionContext ctx) {
         );
     }
 
+    @Test
+    void doesNotAddSemanticallyEqualDuplicate() {
+        rewriteRun(
+          spec -> spec.recipe(toRecipe(() -> new XmlVisitor<>() {
+              @Override
+              public Xml visitDocument(Xml.Document x, ExecutionContext ctx) {
+                  doAfterVisit(new AddToTagVisitor<>(x.getRoot(), Xml.Tag.build("<bean id=\"myBean\"/>")));
+                  return super.visitDocument(x, ctx);
+              }
+          })),
+          xml(
+            """
+              <beans>
+                <bean id="myBean"/>
+              </beans>
+              """
+          )
+        );
+    }
+
+    @Test
+    void doesNotAddDuplicateIgnoringAttributeOrder() {
+        rewriteRun(
+          spec -> spec.recipe(toRecipe(() -> new XmlVisitor<>() {
+              @Override
+              public Xml visitDocument(Xml.Document x, ExecutionContext ctx) {
+                  doAfterVisit(new AddToTagVisitor<>(x.getRoot(),
+                          Xml.Tag.build("<bean class=\"C\" id=\"myBean\"/>")));
+                  return super.visitDocument(x, ctx);
+              }
+          })),
+          xml(
+            """
+              <beans>
+                <bean id="myBean" class="C"/>
+              </beans>
+              """
+          )
+        );
+    }
+
+    @Test
+    void addsSemanticallyEqualDuplicateWhenAllowDuplicatesTrue() {
+        rewriteRun(
+          spec -> spec.recipe(toRecipe(() -> new XmlVisitor<>() {
+              @Override
+              public Xml visitDocument(Xml.Document x, ExecutionContext ctx) {
+                  if (x.getRoot().getChildren().size() == 1) {
+                      doAfterVisit(new AddToTagVisitor<>(x.getRoot(),
+                              Xml.Tag.build("<bean id=\"myBean\"/>"), true));
+                  }
+                  return super.visitDocument(x, ctx);
+              }
+          })),
+          xml(
+            """
+              <beans>
+                <bean id="myBean"/>
+              </beans>
+              """,
+            """
+              <beans>
+                <bean id="myBean"/>
+                <bean id="myBean"/>
+              </beans>
+              """
+          )
+        );
+    }
+
+    @Test
+    void addsWhenChildrenShareNameButDifferentAttributes() {
+        rewriteRun(
+          spec -> spec.recipe(toRecipe(() -> new XmlVisitor<>() {
+              @Override
+              public Xml visitDocument(Xml.Document x, ExecutionContext ctx) {
+                  doAfterVisit(new AddToTagVisitor<>(x.getRoot(), Xml.Tag.build("<bean id=\"myBean2\"/>")));
+                  return super.visitDocument(x, ctx);
+              }
+          })),
+          xml(
+            """
+              <beans>
+                <bean id="myBean"/>
+              </beans>
+              """,
+            """
+              <beans>
+                <bean id="myBean"/>
+                <bean id="myBean2"/>
+              </beans>
+              """
+          )
+        );
+    }
+
     @Issue("https://github.com/openrewrite/rewrite/issues/1392")
     @Test
     void preserveNonTagContent() {